#div1{
    width: 20em;
    height: 10em;
    border: black solid 1px;
    background-color: red;
}
/*相对定位*/
#div2{
    width: 20em;
    height: 10em;
    border: lawngreen dashed 1px;
    background-color:white;
    position: relative;
left: 1em;
    top: 1em;
}
/*绝对定位*/
#div3{
    width: 20em;
    height: 10em;
  border: blueviolet dotted 1px;
    background-color: blue;
    position:absolute;
    left: 1em;
    top: 1em;

}
.fixed-side{
    width: 20em;
    height: 10em;
    border: brown double 1px;
    background: bisque;
    position: fixed;
    left: 30em;
    top: 10em;
}
.one,.two,.three{

    width: 1em;
    height: 30em;
    border: black solid 1px;
    margin-bottom: 10em;
    overflow:hidden;
    /*overflow:hidden*/
    float: left;

}
.clf{
    clear: both;
}
.four{
    width: 30em;
    height: 2em;
    border: black solid 1px;
    background-color: crimson;
    margin-bottom: 3em;
}